1. Field of the Invention
The present invention is concerned generally with detecting and identifying the naturally occurring radioactive elements in earth formations penetrated by a well borehole. More particularly, the invention concerns methods and apparatus for detecting and identifying elemental concentrations of potassium, uranium, and thorium in earth formations in the vicinity of the well borehole by analysis of naturally emitted gamma rays caused by radioactive decay of these substances.
2. Description of the Prior Art
In recent years gamma ray spectroscopy of earth formations in the vicinity of a well borehole has been made practical by the development by highly stable scintillation detectors which may be lowered into a well borehole and which produce a pulse height spectrum proportional to the gamma ray energy spectrum of gamma rays impinging upon the scintillation crystal. At the present time, two commercial well logging services are available for detecting the natural gamma ray spectra produced by uranium, potassium, and thorium in earth formations in the vicinity of a well borehole.
In the first of these commercially available services three energy ranges or windows centered about selected gamma ray emission peaks for naturally occurring gamma rays in the decay chain of the aforementioned elements are selected. Gamma ray count rates in each of these three energy ranges are transmitted to the surface and processed by a technique called spectrum stripping wherein standard calibration spectra for each of the individual elements (made in standard boreholes) are applied to the measurements of count rates made in the energy ranges chosen to detect each of the three elements sought to be detected. So called "stripping constants" derived from the measurement of the standard spectra in standard boreholes for each of three elements are then applied to the measured spectrum in the unknown earth formations surrounding the borehole. An estimate of the percentage of the particular three elements desired to be detected is made based on the stripping technique.
In a second, slightly more sophisticated commercially available technique, (such as that described in U.S. Pat. No. 3,976,878 to Chevalier, et al issued Aug. 24, 1976) five energy ranges or windows are utilized for the measurement of the spectrum of natural gamma rays emitted by the earth formations in the vicinity of a well borehole. The five energy range measurements used in this technique are employed in a least squares fitting scheme to determine the elemental concentrations of the three elements, again based on the spectrum of each of the individual elements of the three taken in standard boreholes. The use of the five windows gives an overdetermined set of equations (i.e. 5 equations in 3 unknowns) which are statistically enhanced by the use of the count rate data from the extra two energy windows to distinguish this technique from that described in the foregoing paragraph.
A problem which occurs in the use of either of the above referenced prior art techniques is that the measurements of the gamma ray spectra of the unknown earth formations in each instance are compared with standard spectra for each of the individual elements made in standard borehole conditions. That is to say, standard condition borehole spectra are fitted to the spectra obtained in the unknown borehole being measured in each of the above two techniques. The problem arises due to the fact that the borehole conditions in the unknown earth formations being measured may not correspond to those of the standard, or calibration, spectra boreholes. That is to say, if the standard spectra were made with a scintillation detector placed in an eight inch borehole, the effect of the borehole dimensions or casing and mud conditions on unknown spectra taken in, for example, a six or twelve inch borehole could cause variations in the spectrum stripping or fitting techniques. These variations could effect the estimates of the percentage of the three elements sought to be detected by as much as an order of magnitude. The present invention takes into account the differences in borehole sizes and borehole effects from the standard gamma ray spectra used for the comparison to the unknown spectra taken in the unknown borehole of interest.